/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution
{
    long prev = LONG_MIN;
public:
    bool isValidBST(TreeNode* root)
    {
        if (root == nullptr) return true;

        bool left = isValidBST(root->left);
        bool ret = false;
        if (prev < root->val)
        {
            prev = root->val;
            ret = true;
        }
        bool right = isValidBST(root->right);

        if (left && ret && right)
        {
            return true;
        }
        else
        {
            return false;
        }

    }
};